Cathode-ray tube magnetic focusing



y 1951 R. E. MEAGHER 2,555,147

CATHODE-RAY TUBE MAGNETIC xocusmc Filed Sept. 14, 1945 I I I3ACCELERATING ANODE VOLTAGE J FOOUSING COIL 6 8 7 /REGULAT ED VOLTAGE 4 5ATTORNEY.

Patented May 29, 1951 CATHODE-RAY TUBE MAGNETIC FOCUSING Ralph E.Meagher, Watertown, Mass., assignor, by mesne assignments, to the UnitedStates of America as represented by the Secretary of War ApplicationSeptember 14, 194.5, Serial No. 616,374

4 Claims.

The invention relates to an electrical circuit for keeping the electronbeam of a cathode ray tube in focus, substantially independently ofvariations in potential on the accelerating anode of the tube caused forinstance by variations in the line voltage or of variations in theresistance of the focusing coil due to temperature changes.

It has been found that, in cathode ray tubes using magnetic typefocusing and deflection apparatus for focusing and deflecting theelectron beam, images that have been expanded for projection becomeseriously distorted, due to defocusing, with even a small deviation fromthe proper current in the focusing coil. Even the change in resistancedue to temperature change of the coil is sufficient to cause defocusing.The critical state of the focus makes it necessary for the focus controlknob to be readjusted continually to compensate for small unavoidablechanges in the A.-C. voltage from which the high positive potential forthe accelerating anode is obtained. It is also necessary to makeadjustments to compensate for the change in resistance of the focusingcoil as the apparatus warms up. Constant manual readjustment isundesirable and, in addition, is impractical. It is therefore an objectof this invention to provide a circuit which compensates for voltagevariations and temperature changes and which eliminates the need forreadjustment of the focus knob once it is set initially.

With cathode ray tubes used in radio detecting and ranging equipment itis essential that the pattern on the screen be clear and undistortedsince measurements are made from the screen and any distortion or lackof clarity would introduce inaccuracies. It is also important, forobvious reasons, in cathode ray tubes used for television to keep theimage clear and undistorted.

The problem does not arise in cathode ray tubes using electrostatic typefocusing for the electron beam, since the voltage required for focusingvaries directly with the voltage on the accelerating anode and thereforesmall changes in line voltage will not cause deiocusing.

In a cathode ray tube using magnetic type deflection and focusing butwith electrostatic acceleration, current in the focusing coil shouldvary as the square root of the potential on the accelerating anode ifthe beam is to be kept in proper focus. The reason for this is that theeffect of magnetic flux on the electron beam depends directly on theflux density which in turn depends directly on the current through thecoil. The effect of the accelerating potential on the electron beamvaries inversely as the square root of the potential. If the focusingcoil is connected to a source of potential which varies in the samemanner as the accelerating potential, the current through the coil,neglecting the change of resistance in the coil due to temperaturevariation, will vary directly with the accelerating potential anddefocusing will take place.

Another cause of defocusing is the variation of the resistance of thefocusing coil with changes in the temperature of the coil due to changesin ambient temperature and the warming up of the apparatus in which thecathode ray tube is installed. It is therefore a further object of theinvention to provide a circuit which will cause the current through thefocusing coil to vary as the square root of the accelerating potentialdespite variations in the resistance of the coil.

A further object is to provide a circuit that will keep the fiux densityof the focusing field of a cathode ray tube proportional to the squareroot of the accelerating potential.

Other objects and advantages will become apparent from the hereinafterdescribed specification.

The current for the focusing coil is supplied through a pentode typevacuum tube. The voltage supply for the screen is obtained from aregulated source of positive potential. The cathode of the tube isconnected to ground through an adjustable resistor. The grid bias forthe tube is obtained from a source of positive potential which variessubstantially as the square root of the accelerating anode potential.The current through a pentode type vacuum tube depends almost entirelyon the grid bias, over a wide range of anode potential, if the screen isheld at a constant potential. ihe current through the focusing coil willtherefore be substantially independent of variations in focusing coilresistance and in the focusing coil voltage supply and will depend onthe bias on the grid of the tube. The bias on the grid of the tubevaries as the square root of the accelerating voltage and the focusingcoil current will vary in the same manner, keeping the focus independentof small variations in line voltage and of variation in the resistanceof the focusing coil.

The invention will be best understood by reference to the single figureof the drawing which shows one embodiment thereof.

Point A is connected to the same source of high positive potential asthe accelerating anode of the cathode ray tube. Resistors 2 and 3 areconnected between point [and ground. These resistors are so proportionedthat the potential at point 4 is approximatel the same as that of thesource of regulated voltage (not shown) connected to point 5. Resistors6 and l are of equal value and are connected in series between points :3and 5. At the common junction of the two resistors (point B) oneterminal of a potentiometer is connected with the other terminalconnected to ground. The movable contact of the potentiometer isconnected to the control grid of the vacuum tube 10. The screen grid ofthe tube is connected to point (the source of regulated potential). Thecathode of tube III is connected to ground through an adjustableresistor l I. The anode of the tube is connected through the focusingcoil I2 to an unregulated source of positive potential (not shown)connected to terminal I3.

Point I will be at the same potential as the accelerating anode and willvary in the same manner. Point 4 will be at a lower potential, due tothe voltage divider action of resistors .2 and 3, but will vary in thesame manner as the accelerating anode potential. Point 8, the midpointof the voltage divider connecting points 5 and 8, will have a percentagevariation equal to one half the percentage variation at point t.

The potential at point 5 is kept constant by any well known means, suchas a voltage regulator tube. If the percentage variation at point 8 isone half the percentage variation at point t, the

potential at point 8 will vary substantially as the square root of thepotential at point i, if the variations are not too large. The screengrid of the tube It is kept at a constant potential. The control grid isconnected to the potentiometer 9 and the anode current of the tube andconsequently the focusing coil current, will vary in accordance with thevariations in potential at point 8'. The initial focus adjustment may bemade by adjusting the potentiometer 9. Adjustment could also be made bymeans of the adjustable cathode resistor l I.

It can be seen that the above described circuit will cause the currentthrough the focusing coil to vary substantially as the square root ofthe accelerating anode potential regardless of variations in theresistance of the focusing coil.

It will be apparent that there may be deviations from the invention, asdescribed, which still fall fairly with-in the spirit and scope of theinvention. For example, any other type of circuit may be used to providea potential proportional to the square root of the accelerating anodepotential.

Accordingly I claim all such deviations which fall fairly within thespirit and scope of the invention as identified in the hereinafterappended claims.

What is claimed is:

1. In an electrical circuit, a cathode ray tube having an acceleratinganode and a magnetic focusing coil, a first source of positive potentialconnected to said anode, a first resistance means connecting said firstsource of positive potential to a first point of positive potential, asecond source of reference potential, a second resistance meansconnected between a tap on said first resistance means and said secondsource of positive potential, a vacuum tube having at least three gridsincluding a screen grid, an anode and a cathode, means electricallyconnecting the screen grid of said tube to said second source ofpotential, a potentiometer connecting the midpoint of said secondresistance means to said point of reference potential, meanselectrically connect i'ng a control grid of said tube to saidpotentiometer, a third source of positive potential, and means includingsaid focusing coil electrically connecting the anode of said vacuum tubeto said third source of positive potential.

2. In an electrical apparatus including a source of high potential, asource of regulated potential, a source of unregulated potential, and acathode ray tube having an electron-emitting cathode, an anode connectedto said high potential source for accelerating electrons emitted fromsaid cathode, and a focusing coil for said beam; a circuit for supplyinga current to said focusing coil which varies as a square root of saidhigh potential, for maintaining the focus of said beam in spite offluctuations of said high potential and resistance changes of saidfocusing coil; comprising a first resistor, having a tap thereon,connected between said high potential source and ground; a secondresistor connected between said regulated potential source and said tapon said first resistor, said second resistor having a midpoint tapthereon; a third resistor having an adjustable tap thereon connectedbetween said midpointof said second resistor and ground; means forsupplying a current to said coil through a constant current impedancemeans; and means connected to said movable tap for controlling thecurrent through said impedance in accordance with the square root ofsaid high potential.

3 A circuit according to claim 2, wherein said constant currentimpedance comprises a pentodetype vacuum tube having its screen gridconnected to said regulated potential source, its plate connectedthrough said focusing coil to said source of unregulated potential, andwherein its control grid connected to said movable tap comprises saidcontrolling means.

4. A circuit according to claim 3, further including a cathode biascontrol for said pentode for regulating the current through saidfocusing coil.

RALPH E. MEAGl-IER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,255,485 Dome Sept. 9, l9ll2,291,682 Blumlein et al. Aug. l, 1942

